ENEE-698E

1
(No Transcript)
2
ENEE-698E

• 2nd presentation by
• Saeed Esmaili Sardari
• November 06, 2007

3
Oxygen sensing characteristics of individual ZnO
nanowire transistors

• Q. H. Li, Y. X. Liang, Q. Wan, and T. H. Wanga
• Institute of Physics, Chinese Academy of Sciences
• Beijing, 100080, China
• APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 26 27
  DECEMBER 2004

4
Outline

• Recap of previous presentation
• ZnO nanostructures
• Fabrication of individual ZnO nanowire FET
• Theoretical analysis
• Response to oxygen
• Response to illumination
• Experimental results
• Discussion
• Summary

5
PEFET and nanoforce sensor based on a single ZnO
nanowire
6
ZnO nanostructures

• ZnO has the richest family of nanostructures
  among all materials
• Specific growth conditions lead to different
  structures with different properties
• Nanowires/Nanotubes
• Nanobelts
• Nanocombs
• Nanosprings
• Nanocages

7
ZnO nanostructures

• Three types of fast growth directions along with
  polar surfaces due to atomic terminations give us
  tuning parameters to get different structures

8
ZnO nanostructures
9
Fabrication of individual ZnO nanowire FET

• ZnO nanowires fabricated with thermal evaporation
  of ZnO powders by flowing a carrier gas with an
  oxygen concentration of 20 at 1100 degrees
  Celsius
• Gold electrodes deposited by e-beam deposition on
  a thermally oxidized highly n-doped silicon
  substrate
• Au electrodes are 50 nm thick
• Oxide layer is 500 nm thick
• Nanowires are dispersed with sonication in
  ethanol
• The spacing between electrodes is about 1 micron

10
Fabrication of individual ZnO nanowire FET
11
FET characteristics

• n- or p- type
• Carrier mobility
• Carrier density
• Threshold voltage
• Transconductance
• Gate capacitance

12
FET characteristics
13
FET characteristics

• n- type FET
• VTH -6.2 V
• Gate capacitance
• The gate capacitance was estimated to be 6x10-17
  F from the formula CG2pee0L/ln(2h/r)
• Transconductance from the slope of the curve 79
  ns
• Mobility 6.4 cm2/Vs
• From dI/dVGµ(CG/L2)VSD
• Carrier concentration 2300 µm-1
• from the formula enCLVTH

14
Theoretical analysis

• Oxygen sensing
• UV illumunation
• Ambient oxygen get adsorbed on the surface of the
  nanowire and forms an ionic specie ( O-, O2- ,
  O2- ) The electron is captured from the nanowire
  thus, reduces the carrier density, and increases
  the resistance of the channel
• UV illumination generates electron/hole pairs
  where holes make the adsorbed ionic species
  desorb and increases the conductance of the
  channel

15
Reported results
16
Reported results
17
Discussion

• Experimental results confirms the hypothetical
  theory
• Chemistry of the adsorption and the very fine
  details of the process are not as clear as
  regular chemical reactions
• Different models, and different theories are
  suggested for the resistance/conductance change

18
Summary

• An individual ZnO nanowire FET is presented
• Transistors characteristics are examined under
  different oxygen pressures
• Change in drain-source current
• Change in threshold voltage
• Sensing property of the FET is attributed to the
  capture and/or release of nanowire carriers due
  to adsorption and/or desorption of oxygen at the
  surface of the sensing wire

19
Questions

• Thanks